Hot cathode device



June 14, 1932. A. w. HULL 1,363,407

Ho'r CATHODE DEVICE Filed April 14, 1930 Fig. l.

Inventor: Albertw. HUI l,

Hi; Attmey.

Patented June 14, 1932 UNITED STTES PATENT OFFICE ALBERT W. HULL, OF SGHENECTADY, NEW YORK, ASSIGNOR T0 GENERAL ELECTRIC COMPANY, A. CORIORATION OF NEW YORK Ho'r cA'rHoDn Application filed April 14,

The present invention relates to electrical discharge apparatus, more particularly to thermionic devices containing an ionizable medium and provided with a source of electrons, a cooperating anode and an electrostatic control member (or grid). The pressure of the medium and the impressed voltages are such that a discharge of arc-like character is produced between the electrodes, the initiation of the discharge being con-` trolled by the bias onthe grid. After the arc is started, the grid loses control and can neither modulate, limit nor extinguish the arc. The starting of the arc may be repeated indenitely because while the discharge cannot be extinguished by the grid, it can be stopped by removing. the anode voltage. Upon reapplying this voltage, the grid again determines whether the arc will start and by a continued repetition of this process, the grid can control the average plate current over a period of time. A practical method of obtaining interruption oi the plate circuit is to employ alternating current although it will be understood that direct current may also be utilized in case proper circuit interrupting means are employed. Devices of this character have been described in my article entitled Hot cathode thyratrons in G. E. Review vol. 32, No. 2, April 1929, pp. 213-223 inclusive. Such devices nd utility in circuits where large currents are to be controlled by the application of a very small amount of energy to the grid. The source of electrons may be constituted of a filament which conveniently is energized through a voltage step-down transformer connected to the source of plate current in the case of an alternating currentsupply. lVhen direct current is utilized, a" separate filament energizing battery may be provided for this purpose. The present invention is directed more especially to the configuration of thel iilamentary member, also to the relative position and arrangement of the member with res ect to the remaining electrodes.

n the prior art devices, it has been proposed to utilize M-shaped and inverted V-shaped filaments presented lengthwise tothe anode. However, when a suitable nega- 1930. Serial No. 444,094.

tive voltage is applied to ,the contrel'f'irieiii-f.

ber with respect-to the- -}Jo in t: of? average potential on a filament energized by alternating current, it has been found that the grid is at a positive potential with respect to other portions of the ilament.' This condition produces a steady stream of electrons between the filament portions and the grid, giving rise to an undesired resistance load on the grid controlling circuit. It will be understood 'that in the initiation lof arc discharge, it is desirable to have the grid respond to variations of voltage and not current, particularly in case the power available for control is relatively small, as for eX- ample'when the device is preceded by a photoelectric tube. AWhen the grid-to-'cathode path has al'ow initial impedance as may be caused by an electron iiow between these electrodes, variations of voltage on the grid exercise only a small control and in extreme cases practically no control whatever of the initiation of the arc unless the voltage stimulus is accompanied by a relatively large current.

An object of my invention is to provide an electrode construction and arrangement which will lend to an electrostatically-controlled arc discharge rectifier of the filament type the property of being operated strictly by variations of voltage on the grid. Otherwise stated, the object is to reduce the resistance or impedance load on the grid circuit of a device of the type indicated. Other objects and features will be apparent as the specification is perused in connection with the accompanying drawing in which F ig. l is an elevational view, partly broken away, of a device embodying the principles of my invention; Fig. 2 illustrates a preferred method of control o the device shown in Fig. 1; While Fig.

3 shows two tubes in diagram to assist in eX- plaining the present invention.

In the drawing, numeral 1 designates an evacuated receptacle or envelope terminating in a combined reentrant stem and press arrangement 2. The envelope preferably is of the tipless type which may be evacuated through a tube 3 communicating with the interior. The envelope is seated in the usual base 4 and contains a plurality of cooperating electrodes. The electrostatic control member or grid 5 is supported from the reentrant stem by means of stay rods 6 which are afiixed to a band 7 clamped about the stem. The grid is constituted of mesh material, preferably nichrome (80% nickel, 20% chromium) ,Y as a cylinder positioned longitudinally in the envelope, the cylinder being closed at one end and open at the other end to receive the cathode in the manner shown. A lead wire 8 connects the clamp member to a contact pin 9 aixed in the base 4. The lamentary cath -preferably of carbonized nickel and formed as a dish with the concaved end presented to the other electrodes. The anode is suspended within the envelope by a rigid conductor 16 which connects with an external contact member 17. An insulating sleeve 18 'of glass may be fitted about the conductor 16 to increase .the leakage distance between the anode and the remaining electrodes. The envelope 1 contains an ionizing medium, e. g. mercury vapor or an inert gas such as argon, helium or neon at a pressure sufficiently high to support an arc-like discharge at the operating potentials. The gas pressure should be between l and 100 microns when mercury vapor is used and somewhat higher for the other gases. Where the highest eiiiciency is desired, the gas may be an alkali vapor (such as s0- dium or caesium) to which may be added an inert gas to assist in starting. In the case of mercuryvapor, the pressure may be maintained constant by controlling the temperature of a sin le drop of mercury 23 at a remote portion of t e tube.

The simplest method of operating a device of the type described is to impress alternating voltage on the anodeand a periodicallyvarying or an intermittent voltage on the grid. A particularly satisfactory control of the average plate current may be obtained by impressing alternating voltage on both the grid andanode from a transformer 19 provided with a multi-tap secondary and varying the phase of the grid voltage with respect to that of the anode in any well-known manner. In Fig. 2 there are showntwo sets of curves A and B typically illustrating this method of control. In this figure, curve a represents the anode voltage, curve b the critical grid voltage so-called and curve c the voltage actually applied to the grid. The

critical voltage curve b illustratively indicates the negative voltage or bias on the grid remainder of the positive half cycle. In the set of curves marked A, the grid voltage is nearly 180 out of phase with the anode voltage and current starts near the end of the cycle, while in the curves B the` grid voltage is advanced nearly in phase with the anode voltage, and current starts at almost the beginning and flows for practically the whole of the positive half cycle. It is evident that the average plate current over a given number of cycles may be controlled from substantially zero to practically the entire positive half cycle maximum amount. This type of control has been described in an article by me published in the G. E. Review July, 1929, vol. 32, No. 7 at pages 393-4 entitled Hot cathode thyratrons. Control of the initiation of the arc may also be exercised by periodically or intermittently varying the magnitude of the grid bias about an average negative potential, as is well known in the art. l/Vhatever the type of control, it is desirable, in view of the sensitiveness of the control, that the point in each halt cycle at which the arc starts shall be under the positive and exclusive action of the grid and, in case the power available for control is small, this requires that the grid resistance should be small.

As stated hereinbefore, the prior art tubes have usually employed a filament in the form of an M or inverted V with the terminals farther removed from the grid and anode. In this construction, the potential ditl'erence between the more negative end of the filament and grid is not only greater than the potential difi'erence between the apex of-the filament and grid but the latter is also more positive with respect to this negative end, hence electrons may fiow between portions of the electrodes, producing the grid resistance described above. Moreover, in case the grid is so designed that the electric field between the anode and the negative en'd of the filament is stronger than that between the anode and the apex of the filament then because of the remoteness of this negative end from the anode and its proximity to the glass the effect of charges on the glass walls is very pronounced and may modify the grid voltage at which the discharge will start.

In accordance with the present invention, I have devised a novel form of filament which precludes in a substantial degree both of these deleterious effects, i. e., the introduction of grid resistance and the control-modifying action of the charged glass walls. This improvement is brought about by inverting the prior art filament in which case, it will be noted that the most negative terminal of the cathode is nearest the grid and the anode, i. e. the distance between the portions of the electrodes between which there is the greatest potential difference, instantaneous or otherwise, is the shortest. Thus, in Fig. l if the filament l0 is energized by alternating current, the terminal 20 or 21 which is at the greatest potential difference when referred to the anode 15 and the grid 5, is positioned much closer to the last two mentioned electrodes than the lower apex 22 of the V-shaped cathode. It will be apparent that by this arrangement, the grid is negative with respect to all portions of theiilament and moreover the most negative end of the latter is the least removed from the anode. The improvement over the prior art arrangement is most clearly shown by way of diagram such as is illustrated in Fig. 3, wherein exemplary voltages are assigned to the various electrodes in a pair of tubes, the tube on the lett representing the prior art arrangement of electrodes and the right-hand tube illustrating the improvement. Consider the left-hand view, and assume the lament to be energized by 2.5 volts alternating potential (approximately 4 volts maximum) and contained in a tube having a control ratio of 100. The control ratio is the ratio of anode voltage to grid voltage at which the discharge will just start. Hence, with 100 volts on the anode this tube will start when the grid voltage is *l with respect to the middle of the filament and -3 and l respectively with respect to the ends of the filament. It is evident that inasmuch as the grid is positive with respect to one end of the filament (for example the lower righthand end as shown) electrons proceed from this end and travel toward the grid. The flow of current takes place only from one end however due to the fact that the grid is negative with respect to the other end. However, the electrons which manage to strike the grid cause a current flow and give rise to the objectionable grid resistance as stated hereinbefore. On the other hand, consider the arrangement improved in accordance with my inveniton and as indicated in the righthand diagram of the figure wherein the apex of the filament is positioned remote from the anode andthe grid. The latter may be maintained at a normal bias of -1 volt with respect to the more negative end of the filament in order to restrain initiation of the arc. It willbe noted that the grid is now effectively '3 volts with respect to the middle of the filament and 5 volts with respect to the other end. In other words, the grid is now negative to all portions of the filament and the tendency of the electrons to travel from the latter to the grid is markedly reduced. This desirable result is accomplished without necessitating a negative bias on the grid greater than one volt and hence no greater than that employed in the tube having the prior art inverted V-type filament.

It will be apparent that in positioning the more negative end of the filament nearest the anode and grid, not only is greater effectiveness of control secured from voltage-operated circuits, but also the effect of charges on the lass` walls is diminished which otlers the urther advantage of producing greater positiveness of the control. These advantages are obtained without complication of filament structure, as the problem' of supporting the filament is no greater than in the prior art devices.

What I claim as new and desire to secure by Letters Patent ot' the United States, is'.

l. A thermionic device comprising an envelope containing a plurality of electrodes including a V-shaped filament, a plane electrostatic 'control member and an anode, an ionizing medium in said envelope at a pressure suficient to support an arc-like discharge at the impressed voltages, the negative terminal of said filament being positioned nearer the anode and control member than the apex of the V-shaped member.

2. A thermionic device comprising an envelope containing a plurality of electrodes including a iilamentary cathode, a plane electrostatic control member and an anode, an ionizing medium in said envelope at a pressure sufficient to support an arc-like discharge at the impressed voltages, said cathode consisting of a plurality of strands joined together and terminating in one or more pairs of terminals adapted to receive electrical energy, the instantaneous negative terminal of the cathode being disposed nearer the anode than all other portions of the cathode with the exception of the instantaneous positive terminal.

8. A thermionic device comprising an envelope containing a plurality of electrodes including a V-shaped filament, an electrostatic control member and a plane anode, an ionizing medium in said envelope at a pressure sufficient to support an arc-like discharge at the impressed'voltages, both ends of the filament being positioned nearer the anode than the apex of the filament whereby undesired electron emission to the control member is substantially precluded.

In witness whereof, I have hereunto set my hand this 12th day of April, 1930.

ALBERT W. HULL.

CERTIFICATE 0F CORRECTION.

Parent No. 1,863, 407. June '14, 1932.

' `ALBERT W. HULL.

It is hereby certified that error appears in they printed specification of the above numbered patent requiring correction as follows: Page 3, lines 88 and 98, claims 1 and 2 respectively, strike out the words "a plane" and insert the word an, and lines 89 and 99, for "an" read a plane; andthat the said Letters Patent should be read with thesee corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and-sealed this 30th day of August, A. D. 1932.

M. J. Moore, c

(Seal') Acting Commissioner of Patents. 

